1. Field of the Invention
The present invention relates to an image processing apparatus, an image processing method, and a program for use in tomographic image interpretation using X-rays.
2. Description of the Related Art
Although 40 years or more have passed since a diagnostic apparatus that creates a tomographic image using an X-ray was developed in the 1970s, it is still developed and active as a diagnostic technique today. In recent years, tomosynthesis for reconstructing a tomographic image by using projection images obtained using a limited angle has also been performed. Although usually used for mammography, such a method may also be used in the investigation of an implant in an orthopaedic image because tomosynthesis enables the obtaining of a tomographic image without needing a large apparatus such as a gantry or the like and a metal artefact is usually small.
Among various methods of reconstructing a tomographic image from projection images, filtered back projection has been used as a method of mathematically obtaining a tomographic image at high speed and with high precision, and this method is a central reconstructing method even now. According to this method, the projection images are filtered using a reconstruction filter such as ramp filter, Shepp & Logan filter, or the like for amplifying a high frequency and, thereafter, back projection is performed. However, if a substance which largely absorbs X-rays such as a piece of metal exists in the human body, there is a problem in that an overshoot or undershoot occurs in a boundary area between the human body and the high absorber and a streak artefact occurs in the tomographic image. Although the reconstruction by the filtered back projection has also been performed even in tomosynthesis in recent years, a streak artefact by the metal still appears as an adverse reaction.
When the streak artefact occurs, a streaky or linear noise around a specific area of an image occurs, so that the representation of a lesion, organs, or the like on the tomographic image is deteriorated and investigative performance of the tomographic image is remarkably deteriorated. To prevent such deterioration, various reducing methods of the streak artefact which occurs by the high absorber have been considered for a long time.
Japanese Patent Application Laid-Open No. H08-019533 discloses a method whereby a high absorber is computationally removed from a projection image, the removed portion is interpolated, and thereafter, a tomogram is reconstructed. Japanese Patent Application Laid-Open No. 2008-528228 discloses a method whereby a direction-dependent adaptive filtering process is performed to a streak artefact in a tomographic image, thereby reducing the streak artefact. Japanese Patent Application Laid-Open No. 2006-000226 discloses a method whereby a reconstruction filter in which a high frequency is suppressed is used in a high absorber and an ordinary reconstruction filter is used in a low absorber, thereby reducing a streak artefact.
According to the method of Japanese Patent Application Laid-Open No. H08-019533 (mentioned above), since the high absorber is interpolated in another portion, there is a possibility that an unnatural tomographic image is produced. On the other hand, in the orthopaedic field or the like, there is a case where the user wants to confirm a state of an embedded implant or metal plate. In such a case, if the high absorber image information is removed, there is a risk that it becomes difficult to investigate. According to the method of Japanese Patent Application Laid-Open No. 2008-528228, since the filtering process is performed to the tomographic image, there is a case where an image blur occurs or there is a possibility that the suppression of the artefact becomes insufficient. According to the method of Japanese Patent Application Laid-Open No. 2006-000226, if the suppression of the high frequency is insufficient, there is a possibility that the suppression of the artefact becomes insufficient, and on the contrary, if the high frequency is excessively suppressed, there is a possibility that an image blur occurs in a low absorber portion such as an organ or the like.